One of the reasons I've heard was that mature undergrads tend to do slightly worse in the first year because some of them have a steep learning curve - especially those returning to studies after a while, say those who have had to work after post-secondary studies due to financial difficulties. It's been explained that many of them end off catching up in later years.

Their undergrad students probably haven't been studying for a fair few years before starting - so they have to get back into the studying mindset and re-learn all the knowledge they'd forgotten. This would definitely make the start of the degree a lot harder.

On top of that, it's not unusual for mature students to already have family to support or be in a more committed relationship - and this is something they have to commit time and effort to outside of their studies. They may even be under pressure to try and also balance a part-time job alongside all this (or during the holidays, when they could otherwise be revising) in order to support their family, something Cambridge strongly discourages due to the workload.

The assumption isn't strictly true any longer - Wolfson for instance has been slowly climbing the Tompkin's Table, though I do take your general point.

The four graduate colleges you mention are just that: Graduate colleges, though they all accept a varying number of undergraduates. These are often 'non-standard' entrant, such as mature, affiliated or some other 'niche' that means they don't fit the usual 18/19-year-old model.

What is an achievement for an 19-year-old for whom study has been their only care is very different from that which a 30-something year-old student returning to that level from a lengthy break. The standard expected is the same, but because non-standard students come from such varied backgrounds, the curve is steeper (as was mentioned by another poster). These non-standard students by and large do as well as their fellow 20-year-olds by their finals but it might be true to say there's fewer Firsts in the earlier parts, hence the relatively poor performance at Tomkins.

Tompkins is rather for an external, rather than internal audience anyway. I don't remember any students being remotely interested in it!